1. Field of the Invention
The present invention relates to a device that generates an optical code multiplex signal by encoding optical pulse signals for each transmission channel and then multiplexing these.
2. Description of the Related Art
Recently, communication needs are rapidly expanding, due to the spreading of the internet and the like, and in response, high speed, high capacity networks employing optical fibers are being installed. Since the volume of communication is increasing, optical multiplexing technology that bundles together plural communication channels worth of optical pulse signals, and transmits these along the transmission path of a single optical fiber, is being seen as important. Communication channel is sometimes referred to below simply as channel.
As optical multiplexing technology, research being carried out into Optical Time Division Multiplexing (OTDM), Wavelength Division Multiplexing (WDM), and Optical Code Division Multiplexing (OCDM).
Out of these technologies, OCDM has operational flexibility in that there is no limit to the number of optical pulse signals for transmission and reception allocatable to a single bit on the time axis. Furthermore, OCDM has the feature that plural channels can be set for the same time slot on the time axis, or plural channels can set with the same wavelength on the wavelength axis.
OCDM is a communications method that allocates a different code (pattern) to each channel, and extracts a signal by pattern matching. Namely, OCDM is an optical multiplexing technology that encodes an optical pulse signal with a different code for each channel at the transmission side, and employs the same code for decoding at the reception side as that of the transmission side in order to recover the original optical pulse signal.
According to OCDM, when decoding, only optical pulse signals that match the code, when encoding was performed, may be extracted and processed as valid signals. Therefore, optical pulse signals of the same wavelength, or an optical pulse signal of a combination of plural wavelengths, may be allocated across plural channels. Furthermore, according to OCDM, since the same code needs to be used when decoding at the reception side as the code employed for encoding, decoding cannot be performed unless this code is known. Consequently, OCDM is also an excellent transmission method in terms of data security.
In an OCDM communications method using a phase code, the following steps may be performed. First, output from a continuous wave light source at the transmission side is converted into an optical pulse train, and then, based on this optical pulse train, an electrical transmission signal, this being a binary digital signal, is converted into a return to zero (RZ) format optical pulse signal, generating an optical pulse signal to be transmitted. The RZ format optical pulse signal is sometimes referred to below simply as an optical pulse signal. Next, encoding is performed to the optical pulse signal to be transmitted with an encoder, converting the optical pulse signal into an encoded optical pulse signal, and this is transmitted.
At the receiving side, the encoded optical pulse signal is received, and the encoded optical pulse signal is decoded with a decoder set with the same code as the code set in the above encoder, recovering the transmitted optical pulse signal.
Encoding and time spreading are now defined for the purpose of the following explanation. Encoding of an optical pulse train or optical pulse signal by an encoder is converting each of the individual optical pulses configuring the optical pulse train or the optical pulse signal into plural individual optical pulses arranged over the time axis. Conversion of a single optical pulse into plural individual optical pulses arranged over the time axis is called time spreading the optical pulse, and each of the individual optical pulses generated by time spreading is called a chip pulse.
A decoder converts the individual chip pulses respectively into plural individual chip pulse trains on the time axis, and decodes the encoded optical pulse signal by interfering the chip pulses on the time axis that are superimposed, from the chip pulses that have been time spread and generated from the plural individual chip pulses.
Consequently, the encoder and decoder are common from the standpoint of having functionality to time spread a single optical pulse and generate plural chip pulses arranged over the time axis. Except in instances where there is a particular need to discriminate between encoders and decoders in the explanation, there are occasions where optical pulse time spreading device will be used for both an encoder and a decoder, meaning a device that converts an optical pulse into a chip pulse train.
As an optical pulse time spreading device, a generally employed configuration is a Superstructured Fiber Bragg Grating (SSFBG) configured unit Fiber Bragg Gratings (FBG's) and phase shifters, repetitively disposed alternately in a straight line along on optical fiber waveguide direction. Unit FBG here refers to a portion of a continuous FBG where there is not a change in an effective index modulation period, or a portion of a continuous FBG where there is no discontinuity in phase present, midway in an optical fiber.
The FBG sections only periodically modulate the refractive index of the core of an optical fiber, and the geometry is the same as that of optical fibers employed as an optical communications transmission paths in OCDM. Consequently, by employing an FBG as a configuration element of an optical communications device, connecting FBG's to the optical transmission paths is the same as connecting optical fibers together. Connecting optical fibers together is markedly easier than connecting an optical waveguide path other than an optical fiber, such as, a Planer Lightwave Circuit (PLC) or the like, to an optical fiber. This is one of the main reasons that SSFBG's are generally employed as optical pulse time spreading devices.
As described above, the functionality of an encoder and a decoder is common to both, and when employed in an OCDM communications system, the disposed location within the system determines the role. In other words, an optical pulse time spreading device disposed on the transmission side functions as an encoder, and an optical pulse time spreading device disposed on the reception side functions as a decoder.
As a method for generating an optical pulse train, as well as the method of generation by modulating the output a continuous wave light source using an optical modulator, as described above, there is also the method of directly generating an optical pulse train using a mode locked semiconductor laser.
Furthermore, in place of generating a RZ format optical pulse signal and encoding this optical pulse signal, as described above, an optical pulse train may be coded to generate an encoded optical pulse train by modulating the encoded optical pulse train with an electrical transmission signal which is a binary digital signal. In this manner, no matter whether the encoded optical pulse signal is generated using a method of encoding after generating the optical pulse signal, or whether an encoded optical pulse signal is generated by modulating with an electrical transmission signal after encoding an optical pulse train, the same encoded optical pulse signal is generated due to the following reason.
That is, if the RZ format optical pulse signal is denoted D, and a code denoted C, then encoding the optical pulse signal D with the code C is equivalent to deriving the product D×C. Consequently, generating the optical pulse signal D and encoding this optical pulse signal with code C, is equivalent to deriving the product D×C. Further. encoding an optical pulse train, modulating the generated encoded optical pulse train with an electrical transmission signal, this being a binary digital signal, and generating an encoded optical pulse signal, is equivalent to deriving the product C×D. Deriving the product D×C and deriving the product C×D is equivalent to both resulting in generation of the same encoded optical pulse signal.
In an OCDM communications method employing a phase code, the optical pulse signal is dispersed by an encoder over the time axis, according to a given rule set in the encoder, and thereby converted into an encoded optical pulse signal. In such cases, the given rule is determined by the code.
The following method is known as a specific generating method for an OCDM signal in an OCDM communications system. In this method, first, an optical pulse train is generated by a pulse light source of optical pulses periodically arranged over the time axis, then this optical pulse train is divided up into divisions of the number of channels. Then, by modulating the respective divisions of the optical pulse train, a digital optical pulse signal is generated for each of the channels. A multiplexed OCDM signal is generated in this method by multiplexing these digital optical pulse signals with an optical coupler (see, for example, FIG. 5 in “Variable Bit Rate Optical CDMA Networks Using Multiple Pulse Position Modulation”, by Vahid R. Arbab, Poorya Saghari, Narender M. Jayachandran, Alan E. Willner, published in Tech. Dig., OFC'07, 2007, OM06. (Document 1)).
Furthermore, the following method, different from the generating method described above, is known for generating an OCDM signal. In this method, first, a digital optical pulse signal is generated for each of the channels by employing transmitters including a pulse light source and an optical modulator, with the same number of transmitters employed as the number of channels. Then the digital optical pulse signals generated thereby are encoded with different codes for each of the channels, generating encoded optical pulse signals. A multiplexed OCDM signal is generated in this method by multiplexing these encoded optical pulse signals for all of the channels with an optical coupler (see, for example, FIG. 1 in “10 Gbits/s OPTICAL CODE DIVISION MULTIPLEXING USING 8-CHIP BPSK-CODE WITH TIME-GATING DETECTION”, by N. Wada, K. Kitayama, and H. Kurita, published in Tech. Dig., ECOC'98, pp. 335-336, 1998. (Document 2)).
In a conventional device for generating OCDM communications signals, a loss of energy occurs in the optical carrier wave configuring respective optical pulse trains, or optical pulse signals, at the stage at which an optical pulse train is divided into divisions of the number of channels, and at the stage where the optical pulse train, or the optical pulse signal, for each of the channels is encoded by a different code.
Therefore, the intensity of the OCDM transmission signal, which is the communications signal, is weakened. Consequently, in a conventional OCDM transmission system countermeasures are undertaken, such as, amplifying the OCDM transmission signal, by use of an optical amplifier at a stage prior to transmission, or the like. An optical amplifier for amplifying the OCDM transmission signal in Document 1, introduces the section of Erbium Doped Fiber Amplifier (EDFA) of FIG. 5 in order to amplify the OCDM transmission signal, and in Document 2, introduces the section of EDF of FIG. 2 in order to amplify the OCDM transmission signal.
When the OCDM transmission signal is amplified, such as by an amplifier of the like, as well as various technical issues arising, such as the introduction of noise during amplification, the manufacturing costs of such a device and the operational costs thereof are also raised.